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Date:   Mon, 20 May 2019 09:57:31 +0100
From:   Stefan Hajnoczi <stefanha@redhat.com>
To:     Stefano Garzarella <sgarzare@redhat.com>
Cc:     netdev@vger.kernel.org, "David S. Miller" <davem@davemloft.net>,
        "Michael S. Tsirkin" <mst@redhat.com>,
        virtualization@lists.linux-foundation.org,
        linux-kernel@vger.kernel.org, kvm@vger.kernel.org,
        Jason Wang <jasowang@redhat.com>
Subject: Re: [PATCH v2 1/8] vsock/virtio: limit the memory used per-socket
Message-ID: <20190520085731.GA22546@stefanha-x1.localdomain>
References: <20190510125843.95587-1-sgarzare@redhat.com>
 <20190510125843.95587-2-sgarzare@redhat.com>
 <20190516152533.GB29808@stefanha-x1.localdomain>
 <20190517082505.ibjkuh7zibumen77@steredhat>
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On Fri, May 17, 2019 at 10:25:05AM +0200, Stefano Garzarella wrote:
> On Thu, May 16, 2019 at 04:25:33PM +0100, Stefan Hajnoczi wrote:
> > On Fri, May 10, 2019 at 02:58:36PM +0200, Stefano Garzarella wrote:
> > > +static struct virtio_vsock_buf *
> > > +virtio_transport_alloc_buf(struct virtio_vsock_pkt *pkt, bool zero_c=
opy)
> > > +{
> > > +	struct virtio_vsock_buf *buf;
> > > +
> > > +	if (pkt->len =3D=3D 0)
> > > +		return NULL;
> > > +
> > > +	buf =3D kzalloc(sizeof(*buf), GFP_KERNEL);
> > > +	if (!buf)
> > > +		return NULL;
> > > +
> > > +	/* If the buffer in the virtio_vsock_pkt is full, we can move it to
> > > +	 * the new virtio_vsock_buf avoiding the copy, because we are sure =
that
> > > +	 * we are not use more memory than that counted by the credit mecha=
nism.
> > > +	 */
> > > +	if (zero_copy && pkt->len =3D=3D pkt->buf_len) {
> > > +		buf->addr =3D pkt->buf;
> > > +		pkt->buf =3D NULL;
> > > +	} else {
> > > +		buf->addr =3D kmalloc(pkt->len, GFP_KERNEL);
> >=20
> > buf and buf->addr could be allocated in a single call, though I'm not
> > sure how big an optimization this is.
> >=20
>=20
> IIUC, in the case of zero-copy I should allocate only the buf,
> otherwise I should allocate both buf and buf->addr in a single call
> when I'm doing a full-copy.
>=20
> Is it correct?

Yes, but it's your choice whether optimization is worthwhile.  If it
increases the complexity of the code and doesn't result in a measurable
improvement, then it's not worth it.

Stefan

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